Immunocytochemical Localization of the Base Excision Repair Enzyme Uracil DNA Glycosylase in Quiescent and Proliferating Normal Human Cells1

The immunocytochemical localization of the base excision repair en zyme uracil DNA glycosylase was examined as a function of cell prolif eration. Two nontransformed normal human fibroblast cell strains were analyzed using an anti-human uracil DNA glycosylase monoclonal anti body. In quiescent cells, basal levels of a nonnuclear immunocytochemically reactive glycosylase protein were detected. No nuclear immunofluorescence was observed. In contrast, in proliferating cells, intense immunofluorescence could be detected exclusively in the nuclear or perinuclear regions. As proliferation diminished, basal levels of the nonnuclear immunocytochemically reactive glycosylase were again ob served. The subcellular distribution of the glycosylase was examined in parallel by in vitro biochemical assay. In quiescent cells, glycosylase activity was observed in both the nuclear and membrane fractions. A small amount of enzyme activity could be detected in the soluble cytoplasmic fraction. Immunoblot analysis demonstrated a M, 37,000 glyco sylase protein in each subcellular fraction. During cell proliferation, there was an increase in glycosylase activity in each of the subcellular fractions. These results suggest a correlation between the proliferative state of normal human cells and the preferential nuclear or perinuclear localiza tion of an immunocytochemically reactive glycosylase protein. Further, immunofluorescence of the nuclear enzyme may be dependent on defined conformational states of that nuclear glycosylase in the cell cycle.